LABORATORY WORK 165 



narrow jars. The amoebae collect at the surface of the ooze as it 

 deposits, and the layer may be removed with a pipette and the 

 process repeated in a series of test-tubes. If not used at once, some 

 green algae, such as spirogyra, should be added to give oxygen. 



They can often be obtained from dealers. 



Should a source of this Amceba proteus not be at hand, a culture 

 may be made from garden soil by the method described by Goodey 

 (Nature, 25th July 1918). Although most of those obtained are of 

 the smaller kinds, large ones may sometimes be found. The follow- 

 ing somewhat simplified method will serve our purpose. Boil some 

 hay or grass in water. Filter. Neutralise. Place a layer 2 to 3 mm. 

 deep in several Petri dishes. Add to each a gram or so of garden 

 earth. Keep in the light. After two or three days, according to 

 the temperature, amcebcE may be obtained by floating a cover-glass 

 on the surface for a minute or two in order to allow the organisms 

 to fix themselves. Rinse gently with water, and a cleaner prepara- 

 tion will be made than if material is removed in a pipette. It is 

 well to place a short bit of hair on the slide in order to avoid 

 pressure on the organisms. If the surface film is used, a small 

 quantity of the liquid is to be placed on the slide before inverting 

 the cover-glass into position. Numbers of bacteria and ciliate 

 protozoa will be seen, especially if the material from the bottom be 

 taken. If the culture becomes nearly dry, encysted forms of amoebae 

 will be found. The larger ones may be picked out and transferred 

 to fresh culture medium. 



In addition to observing the properties of the protoplasm and 

 the formation of pseudopodia, the nucleus, the contractile vacuole 

 and the food vacuoles should be noted, especially the spherical 

 form of the latter. (See Huxley and Martin's "Practical Biology," 

 pp. 21, 22.) 



For electrical stimulation two strips of tin foil, or better, thin 

 platinum, are cemented on to a microscope slide with Prout's glue, 

 leaving a space of 2 mm. between them in the centre. Each of 

 these is connected to one of the terminals of the secondary coil of 

 an induction apparatus, as described below (p. 205), by means of a 

 fine wire which may be held in contact with the foil by a little lead 

 weight. A weak stimulus must be used to begin with, and gradually 

 increased until an effect is produced. A strong stimulus kills the 

 organisms at once ; their contents pass into the water, and are 

 dispersed therein. Note the significance of this fact as regards the 

 liquid nature of the protoplasm and the necessity for an outer 

 membrane of some kind. 



The Leucocytes of the Blood. If amoebae are not to be 

 obtained, many of the facts of the preceding section can be made 

 out on the colourless corpuscles of the blood. A high magnifica- 



